1. Field of the Invention
The present invention relates to DC—DC converters, and more particularly to a constant frequency duty cycle independent synthetic ripple regulator.
2. Description of Related Art
A single-phase synthetic ripple DC—DC regulator was introduced in a patent application U.S. Ser. No. 10/236,787, filed Sep. 6, 2002, now issued as U.S. Pat. No. 6,791,306, issued Sep. 14, 2004. For the synthetic ripple regulator described therein, ripple current of an output inductor is simulated to generate a synthetic ripple voltage on a ripple capacitor. In general, the voltage applied to the inductor, or the voltage across the inductor, is converted to a current which is applied to a ripple capacitor. One end of the output inductor is coupled to a phase node between switching devices (e.g., FETs or the like) controlled by a pulse-width modulation (PWM) control circuit which switched the voltage of the phase node (by switching the switch devices) between the input voltage Vin and ground based on a PWM signal. The other end of the inductor is coupled to the output node developing the output voltage Vo. In an exemplary configuration, a transconductance circuit converts Vo to a discharge current used to continuously discharge the ripple capacitor and the transconductance circuit converts Vin to a charge current used to charge the ripple capacitor during a portion of each PWM cycle. Thus, the ripple capacitor is charged with gm·(Vin−Vo) current during the PWM ON cycle and discharged with gm·Vo current during the PWM OFF cycle (where a dot “·” denotes multiplication and where “gm” denotes transconductance). A related application, U.S. Ser. No. 10/853,733 filed May 25, 2004, described several variations of the basic synthetic ripple regulator theme.
In various applications, such as in mobile applications or the like (e.g., hand-held unit, laptop computer, etc.), the input voltage Vin has a relatively large range. As an example, in some configurations the input voltage ranges anywhere between 7 and 25 Volts (V), depending upon various factors such as connection of a battery or line voltage, charge condition of the battery, etc. The particular input voltage levels and allowable voltage ranges may vary depending upon the implementation and application. The output voltage may also change depending upon various modes of operation, such as a higher value when operating in a full power mode and a lower voltage value when operating in any one or multiple lower power modes. In one exemplary configuration, the battery voltage may range anywhere between 0.5V and 1V, although other voltage levels and ranges are contemplated. The synthetic ripple regulator provides many benefits and advantages when used for DC—DC regulation of electronic devices including mobile devices. It is noted, however, that the PWM DC frequency of operation of the DC—DC regulator is different with changes of Vin and/or Vo. Although it is acceptable and desirable that the PWM frequency change during load transients and the like, it is desired that the frequency remain relatively the same for each of various DC operating conditions in spite of changes in the input or output voltages.